The overall objective of is to test the hypothesis that hormone replacement therapy (HRT) with estrogen and progesterone will allow specific cardiovascular adaptations to endurance exercise-training that otherwise would not occur in postmenopausal women. The primary purpose old this study is to determine whether HRT alone, endurance exercise- training alone, and/or the combination of HRT and endurance exercise training will reduce arterial stiffness, assessed by pulse wave velocity, in 60 to 72 year old women. In addition, it will be determined whether the combination old HRT and endurance exercise-training enhances cardiac output measured during maximal treadmill exercise and whether this increase is related to a reduction in pulse wave velocity.